The collecting electrodes used at present to create electrostatic field usable for production of nanofibres from polymer solutions and melts are designed first of all as simple sheet-metal, metallic plates. Such electrodes meet the condition for creation of electric field, nevertheless only in terms of quantity. For the production process of nanofibres through the method of electrostatic spinning in a larger than laboratory scale it is essential, that electric field meets also concrete qualitative parameters.
According to DE 101 36 255 A1 the spinning electrode is formed by a system of spinning wires arranged parallel between two mutually parallel endless belts, guided between the upper and lower cylinders, which are arranged one above another. The spinning wires in the lower section extend into a reservoir of polymer solution. Opposite to a section of spinning electrode carrying out the polymer solution from reservoir there is arranged a collecting electrode formed by an electrically conductive circulating belt of wire netting or of metallic foil. A surface of collecting electrode adjacent to the spinning electrode is larger than the respective surface of spinning electrode. Spinning electrode and collecting electrode are connected to opposite poles of the source of high voltage, so that an electrostatic field is induced between them, which serves for spinning of polymer solution carried out into electric field on spinning wires. The produced fibres are deposited on a substrate fabric, which is guided on surface of collecting electrode. At this device, the electric field is induced between individual spinning wires of spinning electrode and the surface of collecting electrode, while the spinning wires move in the direction from the reservoir of polymer solution upwards and an electric field of each spinning wire moves together with it. In this case, the disadvantage is especially the mutual influencing of electric fields of individual spinning wires, because all spinning wires have the same polarity and voltage. On borders of electrically conductive belt or foil forming the collecting electrode so called triple points are formed, a corona is generated and, as a result of it the defects in homogeneity of electric field occur between the spinning and collecting electrode, the defects at forming the fibres in electric field and unevenness in transport of fibres to the substrate material laying on a whole surface of the collecting electrode.
DE 101 36 255 A1 further in the claim 8 and in the paragraph 16 discloses the possibility to use two spinning electrodes, as described above, arranged one against another, and between them, in the position of collecting electrode, there is positioned or guided the fabric. Spinning electrodes have an opposite polarity and the fibres produced on spinning electrodes deposit from each side to one surface of the fabric with opposite charge, which remain bound in the fibres. It is obvious, that electric field for electrostatic spinning is induced between both spinning electrodes and the fibres due to their opposite charges are attracted one to another and deposit on opposite sides of the fabric. Inducing of a homogenous electric field at this embodiment is nearly impossible and according to a current experience the described device either would not be working at all or irregularly and for a very short period.
EP1 059 106 A1 discloses the device for electrostatic spinning of polymer solutions, at which the spinning electrodes are formed by a system of nozzles or a system of discs and the collecting electrode is formed by a conductive endless driven belt, which is grounded. Electric field at this embodiment is induced between the spinning electrodes and a section of conductive endless belt situated against the corresponding spinning electrode. The disadvantages of this embodiment are the same as of the belt-type collecting electrode according to DE 101 36 255 A1 described above.
CZ patent 294 274 discloses the rotating spinning electrode of a cylindric elongated shape. Around a section of circumference of the spinning electrode there is arranged the collecting electrode in a shape of a semi-cylinder made of perforated sheet metal, on whose inner circumference there is guided the substrate material, which is pressed to the inner surface of the collecting electrode due to underpressure in the space behind the collecting electrode. This arrangement is complicated from the point of view of the function, as it is very probable, that during motion of the substrate material this will be taken away from the inner surface of the collecting electrode, and due to this an uneven depositing of fibres will occur on surface of substrate material. At the same time such collecting electrode shows disadvantages in a case if considerably electrically non-conductive substrate or carrying materials are used. Either an electric field induced between the cylindrical spinning electrode and a semi-cylinder collecting electrode will not be homogenous, because in the middle section of the cylindric spinning electrode an electric field will have a lower intensity than on borders, while non-homogeneity will further be supported by occurrence of so called triple points on borders of the collecting electrode, and very probably also on borders of holes for air passage through the sheet metal of the collecting electrode.
Next to this, CZ 294 274 discloses the plate and rod-shaped electrodes, which are due to the spinning electrode positioned behind the substrate material, which does not touch their surfaces. Electric field is induced between the cylindrical spinning electrode and individual rods forming the collecting electrode. Resultant electric field is not homogenous and may be unstable in time. In a course of the process and on the nanofibrous layer this will show itself especially by a drop and increase in irregularity of performance.
To overcome these disadvantages, the collecting electrode according to PV 2006-477 has been designed, which contains a conductive thin-walled body of electrode, in which there is performed at least one opening on whose circumference there is arranged a border, while in an inner space of electrode body there is positioned at least one holder of electrode connected with at least one brace fastened in the spinning chamber, while the holder of electrode is arranged behind the border of opening and is electrically non-conductive.
The advantage of such construction of the collecting electrode is that it does not contain any sharp shapes or shapes with high curvature, and that the points where three differently dielectric solid environments (triple points) are coming into contact, are hidden into the electrode body, where the electric field has zero intensity. Consequently the result is that the electrode does not produce corona and thus an electric field, which is co-induced together with other electric elements, is affected only by the geometry of the electrode. This fact contributes markedly to that the electric field may be much more better adjusted and controlled.
The disadvantage of collecting electrodes according to the background art is first of all a problematic method of creation and deposition of nanofibres and nanoparticles from polymer solutions and melts in cases, when very non-conductive substrate material is used, e.g. electrostatic non-modified hydrophobic polypropylene spunbonds and meltblowns. The relative material and production complexity of these electrodes should be mentioned as well.
The goal of this invention is to suggest a production method of a layer of nanoparticles or layer of nanofibres, which would remove the disadvantages of background art, and thus contribute reliably to creation of defined and stable electrostatic field of a required intensity on process electrodes in areas, where the process of production of nanoparticles from polymer solutions or melts or spinning of polymer solutions or melts is initiated and ran. The invention especially solves the problem with usage of extremely non-conductive substrate materials, because it enables the nanoparticles or nanofibres to be deposited on such materials during electrostatic spinning.
The goal of the invention is also construction of a device for such type of production which would be simple and especially reliable on a long-term basis.